The invention resides in an apparatus for imprinting an unmarked foil with images which are to be disposed within sections of the foil of a predetermined length. The apparatus includes a printing device through which the foil is continuously moved and, after being imprinted, the foil is moved to a transport device in which the foil is advanced in steps of the predetermined length at a time. A sensor generates a printing device activation signal when the length of the foil between a section limit of the foil in the transport device and the printer is a predetermined multiple of the predetermined section length so as to cause the image to be printed properly within a section of the foil. A foil buffer arrangement is disposed between the printing device and the transport device.
Such an apparatus is known for example from Applicant""s U.S. Pat. No. 5,964,151 issued Oct. 12, 1999. The apparatus described therein is used preferably for blister packs having pockets including particular pills wherein the images must be accurately disposed on the pockets.
The pills are disposed in the pockets of blister foils of plastic or paper, which are sealed by a cover foil. The imprint on the cover includes data concerning the pills in the various pockets, for example, instructions as to the use of the individual pills. It is therefore very important that the data are properly located with respect to the location of the pills, that is, on the cover foil on top of the proper pockets. Any mismatch between the cover foil and the blister foil must therefore be minimal, that is, the information should be accurately disposed on the respective pockets which include the pills to which the information applies.
With the known apparatus, the cover foil can be applied to the blister foil with high accuracy. This is achieved essentially by the fact that the beginning of a foil section to be imprinted is constantly re-determined. By the constant re-determination of the beginning of the section to be imprinted, errors are not added up. As a result, even relatively large deviations are generally not detrimental since a deviation on one section does not affect the positioning of the imprint on the next section. Also, slippage of the foil does not affect the following image positions.
The known apparatus is particularly advantageous if the sections are removed from the apparatus in a discontinuous manner, since the signal activating the printing device to imprint a single image is generated for each imprint by a sensor when a compensation roller in a foil buffer loop is disposed in a predetermined position. With the stepwise removal of the imprinted foil sections, the compensating roller moves back and forth over a certain distance along a given path, that is, its position changes depending on the length of the foil between the printing device and the section removal device. Consequently, the compensation roller is very suitable for generating a foil length dependent signal.
For controlling the speed of the printing device the prior apparatus includes a ramp-like proximity sensor, which is arranged adjacent the path of movement of the compensating roller. The proximity sensor is so designed or so arranged that the distance of the compensating roller from the proximity sensor changes continuously with the movement of the compensating roller over an active range. In this way, the proximity sensor generates a signal, which depends on the position of the compensating roller that is on the distance of the compensating roller from the ramp of the proximity sensor. This signal is used for controlling the speed of the printing device.
With the stepwise removal of the foil from the known apparatus, the compensation roller is in constant motion so that the output signal of the sensor changes constantly.
In order to prevent the drive of the printing device from constantly changing its speed in accordance with the output signal of the sensor, an average value of the sensor output signal is formed in the known apparatus. It is not indicated in the prior art patent how such an average value is formed.
In practice, the sensor output signal is averaged by making the drive for the printing device relatively insensitive to signal changes. Then the speed of the printing device does not follow directly the sensor signal, but the drive for the printing device is still subjected to relatively large speed changes, which detrimentally affects the operation of the apparatus.
It is the object of the present invention to provide an apparatus like Applicant""s earlier apparatus wherein, however, the drive for the printing devices operates at a relatively constant speed.
In an apparatus for imprinting an unmarked foil with images which are to be arranged within sections of the foil of a predetermined length L, including a printing device, a foil section transport device for moving the foil stepwise in sections of the length L, a cutting device for cutting the foil into the sections and a foil buffer arrangement accommodating a varying length of the foil between the printing device and the transport device, a foil motion sensor is arranged at the exit of the buffer arrangement for sensing the foil movement out of the buffer arrangement and controlling accordingly the speed of the printing device and a foil length sensor is arranged in the buffer arrangement and providing a print initiation signal when the foil length between the end of the foil in the cutting device and the printing device is a predetermined multiple of the foil section length L.
With the movement sensor at the exit of the buffer arrangement, the average speed at which the foil sections are removed can be determined in a separate control unit in a simple manner and very accurately. If the speed at which the foil is moved through the printing device corresponds to the determined average speed, the same length of foil is supplied to the buffer arrangement as is removed therefrom on the average, although in a stepwise fashion. The length of the foil in the buffer arrangement varies therefore around a certain average value. The constant speed with which the foil is moved through the printing device has a highly positive effect on the image quality and the positioning of the print image on the foil. Furthermore, the constant drive speed makes the printing device more reliable. In addition, the life expectancy of the printing device is greatly increased.
In a particular embodiment of the invention, the buffer arrangement includes a compensating roller which is movably so arranged that its position depends on the length of the foil between a section limit determined with the last sectioning step and the printing device. A sensor is provided which detects the position of the compensating roller. The compensating roller is disposed in a loop of the foil so that the foil extends around the compensating roller. If the length of the foil in the buffer arrangement becomes smaller, the compensating roller is moved in the direction of the opening of the loop. If the length of the foil in the buffer arrangement becomes greater, the compensating roller moves away from the open end of the loop so that the loop becomes elongated taking up the increased length of the foil in the buffer arrangement.
With the compensation roller, the buffer arrangement is provided in a simple manner. In addition, the compensation roller forms an element by way of which the length of the foil in the buffer arrangement can be determined in a simple manner. The compensation roller position sensor is preferably an optical sensor and is so arranged that it is disposed in the center of the back and forth movement path of the compensation roller so that the compensation roller passes by the sensor with the same amplitude in both directions.
In another embodiment of the invention, there is provided an offset roller by way of which the length of the endless foil between the respective last sectioning limit and the printing device, and consequently the position of the compensation roller, can be changed. This is then advantageous when the format of the printing image or the size of the section to be imprinted is changed since, in that case, the length of the foil between last section limit and the printing device has to be changed. In order to leave the average position of the compensating roller unchanged inspite of the change of length of the foil in the buffer arrangement, the offset roller is moved to a position wherein the length of the foil between the last section limit and the printing device is again a predetermined multiple of the now changed, that is a second predetermined length L of a foil section, when the compensation roller is disposed adjacent the sensor. In this way, it is not necessary to change the position of the printing device when the length of the foil section is changed.
It has been found to be advantageous if a foil motion sensor, which is preferably a roller rotation sensor is disposed at the entrance to the buffer arrangement. This motion sensor determines the length of the foil and the speed of the foil leaving the printing device and entering the buffer arrangement. Its signal is used for maintaining the speed with which the foil is moved through the printing device constant.
Since the speed with which the foil enters the buffer arrangement corresponds to the speed with which the foil moves through the printing device, the output signal of the foil motion sensor is representative of the speed with which the foil moves through the printing device. If this signal deviates from a foil motion signal provided by a motion sensor sensing the motion of the foil leaving the buffer arrangement or from a desired value provided by a control unit, the operating speed of the printing unit is changed accordingly in a well known manner. The control of the speed of the printing device positively affects the image quality and the accuracy of the printing operation.
With the second foil motion sensor, it is furthermore possible to determine the difference in foil length of the foil entering the buffer arrangement and that leaving the buffer arrangement. A computer in the control unit forms the difference of the signals provided by the first and second foil motion sensors. This signal can be utilized advantageously for a correction of the activation of the printing step as it is in fact used in another embodiment of the invention.
With the correction of the activation of the printing step, the printing process can be started at a point different from the occurrence of the signal provided by the compensation roller position sensor. This permits adapting the printing process to a different length of the print section without moving the offset roller to a different position. The printing process is initiated at a later point in accordance with the change of length of the section to be imprinted. In this way, the offset roller can actually be omitted.
Further features and advantages of the invention will become apparent from the following description of a particular embodiment of the invention on the basis of the accompanying drawings.